Miniaturization and wireless technologies have enabled portable electronic devices to process information in a ubiquitous way.
The more functions and features are integrated into smart phones, for example, the more interaction with the user can be expected. Today's human-machine interface of electronic devices still is strongly relying on key or touch key interaction. However, in many situations such interaction may be difficult since one hand of the user has to hold the device while the other hand may not be free either to interact with the device. Hence, there is a continuous interest to simplify and extend the ways of interacting with electronic devices.